A number of different types of coating systems are available for coating paper webs wherein the coating is applied either to a roll or directly to a paper sheet or the like from a coating chamber opening onto the roll or sheet and having a metering device at the downstream end of the chamber. The metering device generally consists of a blade (i.e. a blade coater) or a rod (metering rod coater). The rod coater uses a rod which may have a contoured surface to meter the amount of coating based, for example, on the size of grooves formed in the rod or in some cases, by using a relatively smooth rod.
To transversely (cross machine) profile, i.e. locally adjust the amount of coating applied at different locations across the machine, it is known to physically deform the blade and/or rod at spaced locations along the blade or rod. Rods are significantly stiffer than blades and thus, are limited in the amount of adjustment that can be made. As the diameter of the rod increases, so does its stiffness which thereby further limits the amount of deformation to which it may reasonably be subjected.
U.S. Pat. No. 4,250,211 issued Feb. 10, 1981 to Damrau, describes a short dwell coater for applying a coating directly to a paper web while permitting uniform bleeding of coating at the upstream side of the coater well to form an air seal at the paper incoming side of the coater.
U.S. Pat. No. 4,706,603 issued Nov. 17, 1987 to Wohlfeil and U.S. Pat. No. 5,078,081 issued Jan. 7, 1992 to Kustermann, shows a coating apparatus with variable area bleed holes all of which are simultaneously adjusted to vary coat weight.
Canadian published application, 2,040,845, published Oct. 20, 1991, inventor, Rantanen, discloses a rod-type coater wherein a large smooth rod is used and wherein its effective flexibility is increased by utilizing different rod structure (hollow) so that the rod may be more easily deflected to permit cross machine direction (CD) profiling of the coating relative to the roll or web being coated.
When metering is performed by a blade, it is possible to profile sections about 7 cm wide by locally deforming the blade. However, with the rod, whether grooved or smooth, profiling is more difficult and is confined to wide sections depending on the stiffness of the rod. It has been found that a conventional rod of about 1 cm in diameter can be deformed for profiling over a length of about 20 cm. But as the diameter of the rod is increased, say to, 3.5 cm, the minimum section length over which it can be deformed is about 50 cm.
PCT patent application, WO93/09290 published on May 13, 1993, discloses yet another form of short-dwell coater wherein a shoe is biased against the web by the pressure of the coating fluid.